Fix the AttributeError message for deletion of a missing attribute

[maggyero]

BPO	43857
Nosy	@rhettinger, @pablogsal, @maggyero
PRs	bpo-43857: Improve the AttributeError message when deleting a missing attribute #25424

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2021-04-15.16:31:31.975>
labels = ['interpreter-core', 'type-feature', '3.8', '3.9', '3.10']
title = 'Fix the AttributeError message for deletion of a missing attribute'
updated_at = <Date 2021-04-18.05:53:26.612>
user = 'https://github.com/maggyero'

bugs.python.org fields:

activity = <Date 2021-04-18.05:53:26.612>
actor = 'rhettinger'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Interpreter Core']
creation = <Date 2021-04-15.16:31:31.975>
creator = 'maggyero'
dependencies = []
files = []
hgrepos = []
issue_num = 43857
keywords = ['patch']
message_count = 1.0
messages = ['391140']
nosy_count = 3.0
nosy_names = ['rhettinger', 'pablogsal', 'maggyero']
pr_nums = ['25424']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue43857'
versions = ['Python 3.8', 'Python 3.9', 'Python 3.10']

[maggyero]

Compare the AttributeError messages in this interactive Python session:

>>> class A:
...     y = 0
...     __slots__ = ('z',)
... 
>>> A().x
[…]
AttributeError: 'A' object has no attribute 'x'
>>> A().x = 1
[…]
AttributeError: 'A' object has no attribute 'x'
>>> del A().x
[…]
AttributeError: 'A' object has no attribute 'x'
>>> A().y
0
>>> A().y = 2
[…]
AttributeError: 'A' object attribute 'y' is read-only
>>> del A().y
[…]
AttributeError: 'A' object attribute 'y' is read-only
>>> A().z
[…]
AttributeError: z
>>> A().z = 3
>>> del A().z
[…]
AttributeError: z

with the `AttributeError` messages in that one:

>>> class B: pass
... 
>>> B().x
[…]
AttributeError: 'B' object has no attribute 'x'
>>> B().x = 1
>>> del B().x
[…]
AttributeError: x

The message AttributeError: x from del B().x does not feel right. I expect this message to be the same as the message AttributeError: 'B' object has no attribute 'x' from B().x, since in both cases the object B() has no attribute 'x'.

I have checked on PyPy 7.3.3 (Python 3.7.9) and it uses the expected message AttributeError: 'B' object has no attribute 'x' from B().x for del B().x. So this confirms my initial suspicion.

----

In CPython, the AttributeError message for attribute retrieval is implemented here (except for slot retrieval):

    if (!suppress) {
        PyErr_Format(PyExc_AttributeError,
                     "'%.50s' object has no attribute '%U'",
                     tp->tp_name, name);
    }

And the AttributeError messages for attribute assignment and deletion are implemented here (except for slot deletion):

    if (dict == NULL) {
        dictptr = _PyObject_GetDictPtr(obj);
        if (dictptr == NULL) {
            if (descr == NULL) {
                PyErr_Format(PyExc_AttributeError,
                             "'%.100s' object has no attribute '%U'",
                             tp->tp_name, name);
            }
            else {
                PyErr_Format(PyExc_AttributeError,
                             "'%.50s' object attribute '%U' is read-only",
                             tp->tp_name, name);
            }
            goto done;
        }
        res = _PyObjectDict_SetItem(tp, dictptr, name, value);
    }
    else {
        Py_INCREF(dict);
        if (value == NULL)
            res = PyDict_DelItem(dict, name);
        else
            res = PyDict_SetItem(dict, name, value);
        Py_DECREF(dict);
    }
    if (res < 0 && PyErr_ExceptionMatches(PyExc_KeyError))
        PyErr_SetObject(PyExc_AttributeError, name);

So it is the last line PyErr_SetObject(PyExc_AttributeError, name); that would be updated. Note that _PyObjectDict_SetItem delegates to PyDict_DelItem (if value is NULL) or PyDict_SetItem (if value is not NULL), and that only PyDict_DelItem can set an exception PyExc_KeyError, which is then translated to an exception PyExc_AttributeError in the last line.

[furkanonder]

@JelleZijlstra Issue seems to be resolved. We can close the issue.